THE DISTRIBUTION OF WATER 19 



the sky. The heat keeps the tiny drops from condensing. 

 Then, suddenly, these high clouds may come into air that is 

 very cold, so cold that it makes their drops condense quickly 

 in the form of ice. The drops of ice plunge down to earth, 

 getting bigger as they fall. These drops of ice we call hail. 



Air Movements. Now you understand how the water 

 gets up into the air. The next thing is to understand how 

 it gets carried over the land. The winds blow the clouds, 

 you say. Yes, but what is it that makes the winds blow ? 

 Air does not move unless there is something that makes it 

 move. What we want to find out is the cause of air move- 

 ments. 



Air is always moving. It is the most actively moving 

 substance in the world. It may move in any direction. 

 Its horizontal movements we call wind. But there are also 

 up and down movements that we do not feel. As it moves, 

 the air carries with it both heat and moisture. You know 

 how important the circulation of the blood is to your body. 

 The circulation of the air is just about as important for 

 the earth as the circulation of the blood is for your body. 

 It is the beating of your heart that keeps your blood mov- 

 ing. What is it that keeps air moving? 



Hot air is lighter than cold air. You have learned about 

 this in geography. You know that the heat of the sun is 

 greater near the equator than it is at the poles. So the 

 warm air near the equator rises and flows away toward the 

 poles, while the cool air from the north and south keeps 

 flowing in toward the equator, under the warmer air. 

 Thus the heat of the sun starts the circulation of earth's 

 atmosphere, just as the heat of a stove starts a circulation 

 of air in a room. 



